As I look at pricing for Canon 70-200L lenses, it looks like the price premium for a 2.8 is about $1,000 (average of IS and non-IS versions). A thousand bucks seems like a lot for basically one stop when new sensors are bring many more times that through rapid ISO improvements (like the 6D). There is so much more room now on ISO to crank it up with very little performance drop off. The notion of "fast lenses" feels more and more like dinosaur film thinking and makes very little sense to me from a value perspective.

The 5D3/6D have a bit over 1/2 stop improvement over the 5D2 only at ISO 1600-3200, and it costs more than $1K more. A fast lens can easily give you much more. But that is not the point - why wouldn't you want to have a more efficient sensor and a faster lens? You can get better IQ in lower light and have acceptable IQ in even lower light.

I'd love to have both, but I guess the reason I wouldn't have both is that these days I can have the 6D + the 70-200 F4 IS lens + keep a $1,000 just by adjusting the ISO on my amazing new sensor...

Are you shooting with a Rebel now? If so, do get the 6D. You gain not because of the better sensor (the new Rebels' sensors are only slightly behind in QE) but because of the difference in the formats. Plus you get better resolution.